1. Field of the Invention
This invention relates to the use of cement plugs in a well, particularly an oil or gas well, and particularly side tracking plugs.
2. Background of the Invention
Rotary drilling of a borehole is accomplished by rotating a drill string having a drill bit at its lower end. Weight is applied to the drill string while rotating to create a borehole into the earth. The borehole may pass through numerous different strata before the borehole reaches the desired depth. The drill string is usually hollow and sections are added to the drill string to increase its length as the borehole is deepened. The trajectory of the borehole is critical to intersecting oil and gas bearing formations. The mechanical variables of the drilling process such as the type of bit, rotary speed, amount of weight applied, and effective stiffness of the drill string have significant impact on the path of the borehole. Additionally, characteristics of the geologic formations traversed by the borehole such as formation dip, formation strike, hardness, fluid content, type of fluid, porosity, permeability, in-situ stresses, compressive and tensile strength, and chemical composition also influence the trajectory of the borehole.
In many instances the design of the drill string, selection of bit, applied weight and rotary speed can be used to control the trajectory of the borehole. However, in some cases, conditions in the drilling operation may require that a new trajectory be established from a point in the existing borehole. Reasons for this include loss of directional control through formation or mechanical variables, failure to locate a target formation at the initially indicated point in the earth, loss of or damage to part of the drill string which causes it to become an obstruction within the existing well path or re-entry into an existing wellbore to increase production from other formations.
Regardless of the reason, altering the path of the borehole is often accomplished by spotting a cement plug or whipstock plug at a preselected point in the existing path. The formulation of the cement plug is such that its hardness is greater than the hardness of the surrounding rock. Hence, the resistance of the cement plug is greater than the resistance of the formation rock and the drill bit will preferentially drill the softer material and establish a new path for the borehole, referred to in the drilling art as "sidetracking".
Setting cement plugs has several potential problems particularly where high strength and good adhesion to the borehole wall are needed in order to divert the path of the drill bit into the surrounding formations. First, contamination of the cement slurry with a drilling fluid generally alters the setting time and compressive strength of the cement formulation. Most water based drilling fluids increase the setting time requiring a longer waiting period for the resumption of drilling operations since compressive strength development is delayed. Oil invert emulsion drilling fluids (oil muds) typically have a high calcium chloride brine internal liquid phase which can significantly reduce the setting time and strength of the Portland cement. Contact with any brine, e.g. seawater, in the drilling fluid or in the well will reduce the strength of Portland cement.
Second, contamination of Portland cement with the drilling fluid is highly probable due to the process used to place the cement plug in the borehole. A successive displacement process is used wherein the cement slurry is pumped down the drill string (or similar work string with an inner diameter substantially smaller than the diameter of the borehole). The slurry is then displaced out the bottom of the drill string into the annulus between the borehole and drill string by pumping a second (displacing) fluid down the drill string. This displacing fluid is typically drilling fluid.
The borehole is filled with drilling fluid and the cement slurry exiting the bottom of the drill string is traveling at a higher velocity than the fluid moving up the much larger annular space. Thus the cement slurry is "jetted" into the drilling fluid as its flow direction changes 180 degrees. Any chemical incompatibility between the drilling fluid and cement slurry may produce a gelled mass that inhibits effective displacement of the drilling fluid by the Portland cement slurry which can result in contamination of the entire cement volume.
Spacers are often used ahead of the Portland cement slurry to prevent contamination of the cement with the drilling fluid. These spacers are similar in composition to the drilling fluid. Clay and/or polymeric thickeners are used to viscosity the base fluid (usually water) in order to suspend weighting agents such as barites, hematites, or ilmenite. Emulsions of oil and water may also be used to provide viscosity for solids suspension. Often, surfactants and/or other solvents may be incorporated into the spacer to improve compatibility with the drilling fluid. Although more chemically compatible with the cement, spacer contamination of the cement slurry can occur and the effect on cement compressive strength is often similar to the effect of drilling fluid contamination.
Third, the cement must adhere to the borehole walls to prevent downward movement of the plug when weight is applied during the drilling operation. This adhesion is typically referred to as the shear bond strength of the cement. Coatings on the surfaces of the formation can reduce the shear bond of the cement. The borehole wall is typically coated with a drilling fluid filter cake which was deposited when the formation was penetrated by the initial drilling operation. This drilling fluid filter cake has low strength and may be sheared off the borehole wall by downward movement of the cement plug during drilling. Drilling may be aggravated by the thickness of the filter cake and any bypassed drilling fluid left in sections of the annulus. Also, any coating of the spacer along the borehole wall may reduce the shear bond strength between the borehole wall and the cement plug.
The density of many materials used for plugs is greater than the drilling fluid density. The primary reason for this is higher compressive strength and greater drilling resistance. For Portland cement plugs, lower water to cement ratios are required to provide greater strength. Greater strength often is used to offset the potential strength reduction due to contamination by the drilling fluid. The disadvantage of high density formulations is the possibility of the plug falling through the drilling fluid below the interval where the plug is desired. The density differential between the drilling fluid and Portland cement increases the probability that an unstable interface will result between the cement and drilling fluid. If the interface between fluids is unstable, cement and drilling fluid can mix causing a poor quality plug.
Accordingly, the present invention is directed to overcoming the above noted problems with Portland cement in the art, and in particular to problems experienced with effective "sidetracking" in oil and gas wells.